Clemens, in U.S. Pat. Nos. 3,842,194, 3,842,217 and 3,909,517, has described a capacitive information disc having video/audio information in the form of geometric variations in a spiral groove in the disc surface. Capacitive variations between the disc surface and a conductive electrode on a playback stylus are sensed to recover the recorded information.
Capacitive information discs may be made from conductive plastic molding compositions which include a vinyl chloride homopolymer or copolymer and sufficient amounts of conductive particles such as carbon black particles so that the disc can provide capacitive readout. The molding may be, for example, by compression, injection or embossing methods.
Capacitive information discs which do not require a grooved surface have also been developed. With this type of disc the stylus is maintained in synchronization with an information track by means of encoded electrical signals on either side of the information track rather than physically by means of the groove.
Because of the high loading of carbon black present with the vinyl chloride-based resin, the melt viscosity of the conductive plastic molding composition is high. As a result, during composition processing, such as extrusion or mixing, high shear stress occurs which causes shear heating that may lead to premature decomposition of the vinyl chloride resin as well as reactions involving the lubricants, processing aids, stabilizers and the like which are present. It would therefore be advantageous to develop conductive plastic molding composition and capacitive information disc records derived therefrom which have increased stability during processing and molding so that decomposition and other reactions which can lead to defects detectable on playback are reduced.